1. Field of the Invention
The present invention relates to a method and an apparatus for dispersing pigment component in an aqueous or oily intermediate mixture in the course of producing a coating composition as a final product, and more particularly to a method and an apparatus for producing an aqueous or oily intermediate mixture wherein the pigment components are uniformly dispersed under reduced pressures.
2. Description of the Related Art
It is known to disperse pigment particles in an aqueous medium or oily medium (sometimes referred to as “a vehicle”) under a reduced pressure to obtain a pigment-containing polymer dispersion useful for producing coating compositions as final products (for example, Japanese Examined Patent Publication No. 46393/1993). Japanese Unexamined Patent Publication No. 351916/2000 proposes a pigment dispersing apparatus for continuously circulating a pigment-containing polymer dispersion between an evacuated tank and a dispersing device to achieve a high dispersing efficiency of pigment.
However, an extremely high vacuum is required in circulating the pigment-containing dispersion between the evacuated tank and the dispersing device under a controlled negative pressure. Such a high pressure necessitates the use of a large-sized vacuum pump with the increasing difficulty of precise control of the pressure. In addition, a seal pump such as gear pump and the like conventionally used for circulating the dispersion tends to suck in air through the sealing portion (axially sealed part) under highly evacuated condition to impair the efficiency of the pigment-dispersing step.
Further, the emission control over volatile organic solvents has recently been tightened. As a result, water-based coating compositions are now widely employed in various fields, replacing the oil-based compositions. However, water-based compositions have an inherent disadvantage that uniform dispersion of pigment particles therein is difficult to achieve compared with the oil-based compositions.
Generally, two types of ball mills are used for the pigment dispersion of an aqueous or oily intermediate mixture useful for producing a coating composition. A ball mill charged with steel balls is conventionally used for the pigment dispersion in oil-based mixture. Use of a steel-ball mill is, however, unavoidably accompanied by inclusion of a small amount of metal derived from the ball media to contaminate the coating composition to be finally prepared and to lower the weatherability of coated film. A pebble mill charged with ceramic pebbles is mainly used for dispersing pigments in water-based composition. However, the pebble mill is low in ability to disperse pigments in the intermediate mixture and the resultant coating composition gives a coating film having poor appearance and lacking surface gloss and evenness.